The invention is directed to a process for the reacquisition of the pitch attitude of an earth satellite which is equipped with an earth sensor and a flywheel that is oriented with its axis of rotation in the direction of the pitch axis (Y axis) of a coordinate system (X, Y, Z) which is fixed with respect to the satellite. The satellite is to be oriented in orbit, with its yaw axis (Z axis) pointed toward the earth'S center and with its roll axis (X axis) pointing in the orbiting direction. The inventive reacquisition process is to be applied in the event that the angular velocity .omega. of the flywheel which is kept between a lower value .omega..sub.L and an upper value .omega..sub.U in normal operation, drops below the lower value .omega..sub.L due to an insufficient energy supply, and earth orientation of the yaw axis is consequently lost.
Geosynchronous earth satellites which are three axis stabilized are frequently equipped with a flywheel and with an earth sensor for the purpose of attitude control. The object of the attitude control is to keep the satellite oriented with the X axis (roll axis) of its coordinate system (X, Y, Z), which is fixed with respect to the satellite, constantly pointing toward the orbiting direction, with its Z axis (yaw axis) constantly pointing toward the earth'S center, and with its Y axis (pitch axis) held perpendicular to the two other axes and relative to the orbital plane. The flywheel is directed with its axis of rotation parallel to the pitch axis, and its angular velocity .omega. in normal operation is kept within a preselectable speed range, defined by means of the lower value .omega..sub.L and the upper value .omega..sub.U. The attitude control around the pitch axis, i.e., the orientation of the yaw axis with respect to the earth, is effected by means of corresponding angular velocity changes of the flywheel as soon as the earth sensor, whose visual field is directed to the earth or the earth horizons, determines a deviation of the yaw axis from the desired orientation to the earth'S center. Accordingly, during one orbital revolution, the satellite rotates approximately once around its pitch axis. Normally, thus, it must constantly have an angular momentum around this axis although this may be relatively small. This angular momentum of the satellite is very small in normal operation in comparison with the angular momentum of the flywheel.
In order to effect corresponding changes in speed of the flywheel during attitude disturbances caused by external influences which are not easily foreseeable, the energy supply of the satellite must be intact. This is usually ensured by means of solar cells and/or batteries. If the energy supply for the engine or motor for changing the speed of the flywheel fails, readjustments during incipient attitude disturbances are no longer possible and the flywheel, also steadily loses rotational energy because of the occurring friction, i.e., its speed soon decreases below the allowable lower value .omega..sub.L. Because of the angular momentum conservation theorem, a corresponding increase in the angular momentum of the satellite occurs, i.e., the latter begins to rotate more than is desired. Therefore, the yaw axis will soon migrate out of the zero or neutral attitude determined by the orientation to the earth'S center, and the earth will disappear from the visual field of the earth sensor. As the speed of the flywheel continues to decrease, the satellite rotates continuously faster. This event can occur particularly when the solar cells are unable to supply energy in the shadow of the earth and the battery fails. When the energy supply is restored, the satellite should be returned as quickly as possible to the desired attitude, i.e., a reacquisition process for the pitch attitude of the satellite must be activated. This process can be introduced, for example, as soon as the direction of solar radiation deviates by not more than 45.degree., for example, from that of the normal line of the solar generator after emerging from the earth'S shadow. This usually happens as soon as the satellite emerges from the earth'S shadow because of its rotation.